1. Field of the Invention
The present invention relates to a numerical, controller for a machining tool that performs a drilling operation.
2. Description of the Related Art
In order to perform a drilling operation of drilling a workpiece, there is known a drilling device that performs a drilling operation by rotating and feeding a tool with respect to the workpiece. As an acceleration/deceleration mode in the drilling operation, a linear acceleration/deceleration mode or a bell-type acceleration/deceleration mode is performed when the tool is decelerated from a constant speed, the tool is stopped from a deceleration state, the tool is accelerated from a stop state, or the tool is changed to a constant speed from an acceleration state. Here, in the linear acceleration/deceleration mode, the tool speed is abruptly changed. Then, in the bell-type acceleration/deceleration mode, the tool speed is changed in a curved state at the early or late time of the deceleration or acceleration, and is changed in a linear state therebetween so that the tool speed is smoothly changed as a whole. It is considered that an impact for a machine may be reduced in the bell-type acceleration/deceleration mode compared to the linear acceleration/deceleration mode.
JP 6-274220 A, JP 2001-312309 A, and JP 2004-202594 A disclose a technique of performing a smooth acceleration/deceleration mode such as a bell-type acceleration/deceleration mode in order to reduce an impact for any machine.
In the above-described related art, the drilling operation is performed at a constant speed. However, since the deceleration is performed in the bell-type acceleration/deceleration mode, a smooth acceleration is performed while the speed waveform in the vicinity of the bottom of a hole is changed from the linear state to the curved state. For that reason, the tool may be stopped at the bottom of a hole so that the tool speed is decreased to about zero while the impact for the machine is reduced. Here, since the next processing is performed after the tool is stopped, the tool is maintained in position at the bottom of a hole in the stop state. As a result, there is a concern that the processing time may be increased.